@media (max-width: 767px) {
    .dropdown-toggle .glyphicon-menu-down {
        color: transparent;
    }
}
@media only screen and (min-width: 768px) {
    li.dropdown:hover > ul.dropdown-menu {
        display: block;
        visibility: visible;
        opacity: 1;
        border-top: 0;
    }
    li.dropdown > ul.dropdown-menu{
        display: block;
        visibility: hidden;
        opacity: 0;
        transition: visibility 0s, opacity 0.3s linear;
    }
}